1. Field of the Invention
The present invention relates to an imaging unit and an image sensor capable of enhancing image quality.
2. Description of the Related Art
In recent years, as multi-functional, high-quality, and compact image sensing apparatuses such as digital cameras and digital video cameras have been developed, high-resolution and super-fine performances are required for image sensors used in the image sensing apparatuses. Thus, a demand for high-pixel and high-density image sensors has been increasing.
A high pixel density image sensor has a drawback that the photoelectric conversion amount per pixel is decreased as the area of one pixel is decreased. As a result, the sensitivity is lowered, and the signal level of a pixel output is lowered, which lowers an S/N ratio (signal-to-noise ratio). The lowered S/N ratio greatly affects image reproducibility and image quality. Accordingly, it is required to enhance the sensitivity in order to realize high-quality image formation. Various approaches have been proposed to enhance the sensitivity in the technical field of image sensors.
In a solid-state image sensor provided with photodiodes arranged in a two dimensional matrix, there is proposed a method for enhancing luminance resolution without an influence of color information concerning an object image (see Japanese Unexamined Patent Publication No. 2003-318375) by providing a filter arrangement that luminance filters Y e.g. a transparent filter or a white color filter, having a large light transmittance, are disposed on checker-patterned photodiodes among the photodiodes of the image sensor to enhance the sensitivity, while conventional color filters of R (red), G (green), and B (blue) are disposed on the rest of the photodiodes to obtain color information, and by using signal charges generated by the photodiodes on which the luminance filters Y having the high sensitivity are disposed, as luminance signals.
In the approach disclosed in the publication, there is a sensitivity difference of three times or more between the pixels (hereinafter, called as “white pixels”) where the luminance filters Y are disposed, and the pixels (hereinafter, called as “color pixels”) where the color filters are disposed. If an exposure control is performed under the same conditions for the color pixels and the white pixels with use of the solid-state image sensor having the above pixel configuration, signal charges generated in the white pixels are saturated, and signal charges generated in the color pixels are exceedingly small. Accordingly, an image signal where a luminance signal and a chrominance signal are kept in a well-balanced state cannot be obtained, thereby making it difficult to form a high-quality image.